Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films, are used in a wide variety of electronic and light electrical components.
Most thick film compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. The binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to the substrate, while the organic medium (vehicle) is the dispersing medium which influences the application characteristics of the composition and particularly its rheology.
One of the most important and widely used class of materials for the conductive phase of thick film resistors are noble metal polyoxides which have the basic pyrochlore structure of A.sub.2 B.sub.2 O.sub.7, in which A is typically bismuth or lead, and B is ruthenium or iridium. In addition, the crystal lattice of this material can also be substituted with other metallic elements. For example, Bouchard in U.S. Pat. No. 3,583,931 discloses the use in thick film resistors of bismuth-containing pyrochlores having the structure (Bi.sub.2-x M.sub.x)(M'.sub.y M.sub.2-y ")O.sub.7-z, in which M is yttrium, thalium, indium, cadmium, lead and certain rare earth metals, M' is platinum, titanium, chromium, rhodium or antimony, and M" is iridium or ruthenium.
A number of U.S. patents to Horowitz et al. disclose pyrochlores of the general formula A.sub.2 [B.sub.2-x A.sub.x ]O.sub.7-y, in which A is bismuth or lead, and B is ruthenium or iridium.
In general, the above-described pyrochlores have been prepared by either of two methods. The first is a solid state reaction, and the second is a liquid phase reaction in an aqueous alkaline medium. Bouchard, U.S. Pat. No. 3,583,931, discloses a solid state reaction process for making the bismuth-containing pyrochlores with the formula given above in which a mixture of the metal oxides or oxide precursors is fired at 600.degree.-1200.degree. C., preferably 750.degree.-1000.degree. C., for from one to 30 hours. Horowitz et al., U.S. Pat. No. 4,124,539, disclose a solid state reaction process for making lead-rich pyrochlores of the formula Pb.sub.2 (B.sub.2-x Pb.sub.x)O.sub.7-y, where 0&lt;x&lt;1.2, in which a mixture of a powdered lead source such as lead nitrate and a powdered ruthenium and/or iridium source, chosen so that the molar ratio of Pb to Ru and/or Ir is at least 1:1 and preferably 1.3:1.0 to 5.0:1.0, is reacted at temperatures below about 600.degree. C. in an oxygen-containing atmosphere.
There are several processes using the liquid alkaline reaction medium (see Horowitz et al., U.S. Pat. Nos. 4,129,525; 4,176,094; 4,192,780; 4,225,469), each of which involves reacting bismuth and/or lead cations with ruthenium and/or iridium cations in a liquid alkaline medium at temperatures below 200.degree. l C. The product produced by these methods has the advantage that it is of desirably small particle size (large surface area), but the methods generally require uneconomically long reaction times.
Thus, while the solid state method with firing at higher temperatures is economical, the surface area of the product obtained is lower than desired. Furthermore, the bismuth pyrochlore generally contains a second phase when prepared by a process similar to the solid state reaction of U.S. Pat. No. 4,124,539. On the other hand, the second method produces pyrochlores of quite high surface area, but the process is considerably less economical. Consequently, there is a real need for a bismuth pyrochlore manufacturing process which is both economical and which results in a high surface area product.